1. Field of the Invention
The present invention relates to a foldable optical apparatus, for example an overhead projector, and more particularly relates to an optical apparatus of improved portability.
2. The Prior Art
There is a known foldable optical apparatus such as an overhead projector, which is advantageous as to portability as it can be folded so as to be of compact size. Such an overhead projector has a removable covering for protection against dust. To use the projector, the covering is first removed. A support or stand is erected and locked in a predetermined erected position above a stage that supports the material whose image is to be projected. The top of the stand swingably supports an image forming head or projector head which incorporates a projecting section and a light source. The projector head, when folded, is contained within the stand. The projector head is substantially parallel to the stage. The stage is constituted by a Fresnel mirror consisting of a Fresnel plate and a reflective layer formed thereon. On the Fresnel plate, there is placed an original of the material, namely a transparency on which an opaque image is written or drawn. To project the image on a screen remote from the apparatus, light emitted by the light source is directed to the Fresnel mirror. Transmitted through the original material, the light is reflected upward by the Fresnel mirror to illuminate the original material from below. The light of the image of the original material passes through a projector lens and to a projector mirror, both carried by the projector head and proceeds to the screen.
This conventional overhead projector has a problem in that removal and attachment of the covering of the apparatus is so laborious and time consuming that it is difficult to set up or store the projector quickly. The covering is so large that use of the projector requires extra space for storing the covering, and this is a drawback when using the projector in a confined space such as the surface of a small desk. Moreover, the size of the covering decreases the portability of the projector, as it adds considerable length, width and thickness to the projector.
The Fresnel mirror has an area the size of the largest original material to be projected. There is a drawback, because when original material smaller than the largest possible is placed on the Fresnel mirror, extra light is transmitted around the original material, so that the stage is excessively lighted and is glaring to the eyes of a user. Moreover, image projected on the screen is surrounded by brilliant marginal areas which are also glaring to viewers of the screen.